The art of forming shaped articles from particulate mixtures is well known in the art. Classically, a desired particulate material is mixed with a binder and then formed into the desired shape, this being called the green body. The green body is then fired to provide a fusion of the particulate material and to drive off the binder, thereby producing the desired shaped product with desired surface texture, strength, etc.
In the production of shaped objects in the manner above described, it has been found that it is necessary to remove the binder before the green body can be sintered. This is a very difficult task and the prior art has recognized this problem and has therefore attempted to remove binder from the shaped green body prior to the step of firing. Examples of said prior art are set forth in the patent of Strivens U.S. Pat. No. 2,939,199, Wiech U.S. Pat. No. 4,197,118, British Patent Nos. 779,242, 1,516,079 and Curry U.S. Pat. No. 4,011,291. Unusual conditions such as vacuum or solvent atmospheres or packing of parts are required by the prior art and cracking of parts due to process irregularities remains a problem. In addition, prior art solvent extraction techniques present a health hazzard which is difficult and expensive to eliminate.
The above noted prior art teaches various methods of removing the continuous phase in such a way so as to minimize the disruption of the green body during the binder removal. In general, the mechanisms involved in the removal of the continuous phase are such that there is interplay between the cohesive forces of the green body, i.e. the tensile strength of the green body, and the forces involved in the removal of the binder. The objective is to maintain, wherever possible, the green body in a net compressive loaded situation and in all cases to never allow the stress forces to exceed the tensile forces of the body, i.e. the forces created by the removal of the binder must always be overcome by the net residual tensile strength of the body, otherwise the body will rupture in some fashion. In addition, as the fluid binder moves across the interface from the interior portions of the body to the exterior environment, viscous drag forces occur which tend to cause exfoliation (a sloughing off) of at least some of the outermost layer of the particulate material on the body, thereby causing loss of material and diminution of the surface integrity of the body which, as is well known, affects several physical properties of material. In a very real sense the physical characteristics of the final sintered body tends to replicate the physical characteristics of the green body. Any macroscopic surface imperfections that are mechanically introduced, such as by physical contact with an external body, would be transferred to the green body and then subsequently, upon sintering, be transferred to the final body. The mechanism of transfer, as would take place, for example, using the wick of Curry U.S. Pat. No. 4,011,291 to the green body would be of two types: the first by the low intrinsic strength of the green body flowing into a scratch due to an imperfection in the wick and the second type would be the viscosity induced flowing of fine particles from the green body into the imperfection area and the subsequent filling of the area by the particulate material of the green body which has been loosened by the flow of material from the body.